1. Field
This disclosure relates to projection displays, more particularly to projection displays using reflective banded color falling-raster illumination.
2. Background
Video projection displays using panels of light valves have undergone rapid growth and expansion in the past few years. These panels, such as liquid crystal displays (LCD) and digital micromirror devices (DMD), generally comprise arrays of individually addressable elements, such as an LCD cell or a mirror. The LCD-based panels may transmit light, where the light passes through the cell, or reflect the light, where the light bounces off of the cell or material directly behind the cell. Generally, except for some instances of image scaling, each element on the panel corresponds to a picture element (pixel) in the final image.
In order to achieve color displays, the panel-based projection systems typically take one of two forms. In the first, one color panel is assigned to each of the traditional three display colors, red, green and blue. However, because of the necessity of having three panels, these systems tend to be larger and more expensive than the other option, which is a one-panel system illuminated with each of the three colors in sequence. In some versions of this architecture, a white light source is used with a segmented color wheel having one segment for each color that spins in front of the light. Each element in the array modulates the colors for the corresponding pixel in sequence, relying upon the integration properties of the eye to blend the three sequenced colors together into a pixel of a particular color.
A disadvantage of this type of approach is that only one color is displayed at a time, resulting in a loss of two-thirds the efficiency of a three-panel system. Advantages include lower cost with fewer panels and no need for the mechanisms to ensure that images from the three panels are aligned correctly, as well as a shorter back-working distance for the optics, resulting in smaller systems.
One method to improve light efficiency scrolls sequential color bands across the panel. An example of this is shown in U.S. Pat. No. 5,410,370, issued Apr. 25, 1995. Typically, a white light source is separated and shaped into red, green and blue bands. Scanning optics, typically consisting of rotating prism blocks with square cross sections, cause the bands to be sequentially scanned across the elements. As a band passes over the ‘top’ of the active area of the panel, a band of that light color also appears at the ‘bottom’ of the panel. Prior to each band of light passing over a given row of elements, that row will be addressed with the appropriate signals to provide the color content of that frame to the corresponding pixels in the image. The image is then projected onto the screen, and the integration properties of the human visual system will integrate it into a complete image. The color bands are moving, and the elements are responding, so quickly that the impression of the viewer is simultaneous full color.
One requirement of a falling raster illumination system is that the color bands (rasters) maintain good uniformity in shape and intensity as they scan down the panel. In order to improve uniformity, current art modifies the surfaces of the rotating block scanning optics to be cylindrically concave. However, this makes the manufacture of the prisms much more difficult and complicated, increasing the costs. Other approaches use an optical medium with varieted thickness, usually a cylindrical, egg-shaped spiral. However, this last approach requires large volume and decreases the efficiency of the optical system.
Additionally, the current art does not achieve uniform illumination across the panel, because the light bands change their dimensions due to variations in the path length of the illumination system to the image plane as the rotating prisms scan the light beam across the light valve. Another difficulty lies in the positioning of the scanning prisms. In one example, the light needs to be focused prior to entering the prism, and requires the use of an extra aperture. This requires relay optics, further raising the complexity of the system.